During the life cycle of memory devices, for example, flash memory devices, such devices are subject to wear and tear due to normal usage. From a signal processing/coding point of view, this implies that the communication channel quality degrades over time, which may eventually affect data reliability of the device. For example, flash memory devices are typically exposed to reliability related issues, such as read disturb, endurance, and retention, as the memory cells cycle through multiple read and/or write operations.
FIG. 1 illustrates the relative threshold voltage distribution of flash memory cells before and after cycling through multiple read and write operations. In particular, FIG. 1 shows the relative voltage distributions before cycling after cycling. The two sets of distributions depicted in FIG. 1 may be interpreted as representations of logic 0 and logic 1, respectively, before and after the memory cells have been cycled.
Notice that after cycling, the separation of the two distributions decreases and each of the distributions become wider. Consequently, the capability for distinguishing whether 0 or 1 is stored in a flash cell degrades after cycling, resulting in data reliability degradation. In the flash memory industry, this phenomenon is often referred to as retention loss. Although an advanced signal processing algorithm may be devised to track the retention loss and mitigate the undesired effects, it may be critical that the degradation of memory cells remains entirely transparent to the user. In other words, from the user's point of view, the data reliability level should be guaranteed throughout the entire lifespan of the device without significant functionality alteration and/or user intervention.